The invention relates to minimizing energy consumption in staged externally-controlled systems.
Staged externally-controlled processes where temperature is the control include distillation processes and chemical reactor processes. Other staged processes can be pressure controlled, such as membrane separation processes like isotope diffusion processes and reverse osmosis processes. Further, staged mechanical controls can apply to staged centrifugation processes.
At the present time, distillation processes account for more than 10% of industrial energy consumption in the United States. Any significant improvement in the efficiency of such processes would result in substantial savings of energy. The traditional fractional distillation column is structured with: (a) one point of heat input (i.e., a reboiler contained in a bottom tray); and (b) a point of heat removal (i.e., a condenser contained in a top tray). Articles suggesting the use of additional heat sources or heat sinks to improve the efficiency of distillation systems include:
1a. xe2x80x9cControl of Sidestream and Energy Conservation Distillation Towers,xe2x80x9d H. A. Mosler, Industrial Process Control (AIChE, NY, 1979);
1b. Conserving Energy in Distillation, T. W. Mix and J. S. Dweck, (MIT Press, Cambridge, 1982);
1c. Distillation Control, F. G. Shinskey (McGraw-Hill, NY, 1984);
2a. xe2x80x9cDistillation with Intermediate Heat Pumps and Optimal Sidestream Return,xe2x80x9d AIChE Jrnl. 32:1347-1359 (August 1986);
2b. xe2x80x9cMinimum Energy Requirements of Thermally Coupled Distillation Systemsxe2x80x9d, AIChE Jrnl. Vol. 33, No. 4, (pp. 643-653, April 1987);
2c. xe2x80x9cHeat Pumps for Distillation Columns,xe2x80x9d A. Meili, Chemical Engineering Progress, 86:60 (1990);
2d. xe2x80x9cEnergy Requirements for Nonconventional Distillation Systems,xe2x80x9d Z. Fidkowski and L. Krolikowski, AIChE Jrnl., 36, 1275 (1990);
2e. xe2x80x9cConsider Thermally Coupled Distillation,xe2x80x9d A. J. Finn, Chemical Engineering Progress, 89, 41 (1993);
2f. xe2x80x9cOn the Use of Intermediate Reboilers in the Rectifying Section and Condensers in the Stripping Section of a Distillation Column,xe2x80x9d R. Agrawal and Z Fidkowski, Ind. Ch. Res., 35:2801-2807 (1996);
3a. xe2x80x9cThermodynamic Analysis of Rectification I and II, Reversible Model of Rectification and Finite Cascade Models,xe2x80x9d Z. Fonyo, Int. Chemical Engng. 14:18 (1974) and 14:203 (1974);
3b. xe2x80x9cThe Ideal Column Concept: Applying Exergy to Distillation,xe2x80x9d V. Kaiser and J. P. Gourlia, Chem. Eng., P. 45 (Aug. 19, 1985);
3c. Industrial Energy Management, V. Kaiser (Institut Francais du Petrole Paris, 1993);
3d. xe2x80x9cEquipartition of Entropy Production: An Optimality Criterion for Transfer and Separation Processesxe2x80x9d, D. Tondeur and E. Kvaalen, Ind. Eng. Chem. Res., V. 26, 50-56 (1987);
3e. xe2x80x9cAnalysis of Entropy Production Rates for Design of Distillation Columns,xe2x80x9d S. Ratkje, E. Sauar, E. M. Hansen, K. M. Lien, and B. Hafskjold, I and EC Research, 34:3001-3007 (1995);
4. xe2x80x9cFinite Time Thermodynamics: Limiting Performance of Rectification and Minimal Entropy Production in Mass Transfer,xe2x80x9d A. M. Tsirlin, V. A. Kazakov, and R. S. Berry, J. P. Chm., 98:3300-3336 (1994);
5a. xe2x80x9cThermodynamic Length and Dissipated Availability,xe2x80x9d P. Salamon and R. S. Berry. Physical Review Letters, 51:1127-1130 (1983); and
5b. xe2x80x9cQuasistatic Processes as Step Equilibrations,xe2x80x9d J. Nulton, P. Salamon, B. Andresen, and Qi Anmin, J. of Chem. Physics, 83, 334 (1985).
The articles listed above are categorized according to technical content in relation to the present invention. Articles 1a-1c are directed to general methodologies of adding heat pumps to effect energy savings from column operation. Articles 2a-2f describe specific examples of these methodologies, but are limited to one additional reboiler and condenser. In some of the examples, a heat pump uses the distillate as its working fluid (vapor recompression). In others, sidestream removal and readdition is used with possible thermal contact outside the column. Articles 3a-3e are directed to examples of multi-tray systems in which the possibility of heat addition and removal at each tray is raised. Their analyses are purely for comparison as idealized aids to analysis of real processes. The specific nature and amount of control at each stage is discussed only in article 3d, which does not present a means for effecting the equal entropy production criterion. This proposition is examined further in article 3e with disappointing results. Article 4 suggests controlling continuous (as opposed to staged) distillation process by adjusting the concentration profile. Articles 5a and 5b describe application of the equal thermodynamic distance principle in a multi-step process for one working fluid traversing a sequence of states.
The approach to heat integration can include the use of a heat pump with at most two points of contact with the column augmented by possible removal and readdition of the distillate material. Prior U.S. patents that address efficiency-improved distillation columns include, for example: F. G. Shinskey, U.S. Pat. No. 4,030,986, xe2x80x9cControl for Maximizing Capacity and Optimizing Product Cost of Distillation Columnxe2x80x9d; G. Emmrich, et al., U.S. Pat. No. 5,080,761, xe2x80x9cMethod of Optimizing the Operation of a Distillation Column Provided with a Side Heating Devicexe2x80x9d; R. Agrawal, et al., U.S. Pat. No. 5,230,217, xe2x80x9cInter-Column Heat Integration for Multi-Column Distillation Systemxe2x80x9d; and R. Agrawal, U.S. Pat. No. 5,289,688, xe2x80x9cInter-Column Heat Integration for Multi-Column Distillation System.xe2x80x9d
The invention is directed to maintaining equal thermodynamic distances between stages in a thermodynamic process. The invention relates, in one embodiment, to a distillation system which minimizes exergy consumption in a distillation column by using heat exchange optimally distributed along the column. The system employs a plurality of thermostatted trays which are maintained at a sequence of temperatures specified by maintaining equal thermodynamic distances between the trays. The system can include a unique employment of heat pumps to attain the desired control. The total heat requirement at the boiler decreases by a factor between two and ten when the temperature of each tray in a column is controlled. The decreased heat requirement is accompanied by a similar reduction in the exergy loss.
In one aspect, the invention features a thermal distillation system. The system includes a plurality of thermostatically controlled trays distributed along a length of a distillation column and a means connected to the trays effective to maintain equal thermodynamic distances between the trays. The means can be a heat flow controller. The means can include a heat pump. The heat pump can be a tandem heat pump, which can be integrated in the design of the distillation column.
The heat pump can include an evaporator, a condenser, a compressor, and a throttling valve connected in series through a conduit. The throttling valve is adapted to circulate a refrigerant to perform a heat exchange cycle. The throttling valve is thermally connected to each of the trays and effective to control the amount of heat transferred to or extracted from the trays. The throttling valve can be an elongated small diameter tube.
In preferred embodiments, the distillation system includes: a distillation column having a bottom tray and top tray; a plurality of intermediate trays disposed between the bottom tray and the top tray, the bottom tray serving as a distillation bottom; a heat source in thermal contact with the bottom tray and effective to supply significant heat energy to the distillation bottom; and a first heat exchanger connected to the intermediate trays. The first heat exchanger can be distributed among the intermediate trays. The first heat exchanger can operate in a reversed Brayton cycle. Alternatively, the first heat pump can include a first conventional heat exchanger. The heat source can be an absorption heat pump and can supply significant heat energy to the column and the absorption heat pump.
The distillation system can also include a heat sink in thermal contact with the top tray and effective to extract significant heat energy from the top tray. The heat sink can include a second conventional heat exchanger. The first and second conventional heat exchangers can be interconnected to form a complete heat pump system.
In another aspect, the invention features a method of minimizing energy consumption in a staged thermodynamic process by maintaining equal thermodynamic distances between each stage in the process. The method can include controlling the amount of heat transferred to or extracted from each stage, controlling the pressure of each stage, or controlling the concentration of a reactant in each stage.
In preferred embodiments, the method includes setting the temperature of each stage in the thermodynamic process. The thermodynamic process can include a first stage and the controlling step can include supplying significant heat energy to the first stage. The thermodynamic process can also include an intermediate stage in which the controlling step includes supplying heat energy to the intermediate stage. The thermodynamic process can further include a last stage, where the controlling step includes extracting significant heat energy from the last stage.
In preferred embodiments, the thermodynamic process is a distillation process.
The invention may include one or more of the following advantages. The system applies the equal thermodynamic distance principle to the steady state operation of a distillation column. This extension is far from obvious and requires mapping the processes in the column to a different transformation of a multicomponent multiphase system. In particular, this required dealing with the xe2x80x9cnull directions problemxe2x80x9d (directions along which the system can move with zero distance traversed and thus with zero dissipation as measured by the geometry) corresponding to changing the amount of any phase (scaling). In addition, the use of additional heat sources or heat sinks can result in possible energy savings.
These and other features and advantages of the invention will be apparent from the following detailed description, and from the claims.